It has conventionally been implemented to measure circuit parameters (S parameters for example) of device under test (DUT). A method for measuring circuit parameters of a device under test (DUT) according to prior art will hereinafter be described with reference to FIG. 20.
A signal with a frequency of f1 is transmitted from a signal source 110 through a DUT 200 to a receiving section 120. This signal is received by the receiving section 120. It is assumed that the frequency of the signal received by the receiving section 120 is f2. The S parameters and frequency characteristics of the DUT 200 can be acquired by measuring the signal received by the receiving section 120.
In the case above, there occur measurement system errors in the measurement caused by, for example, a mismatch between measurement systems such as the signal source 110 and the DUT 200. These measurement system errors are, for example, an error due to the directionality of bridges as Ed, an error due to frequency tracking as Er, and an error due to source matching as Es. A signal flow graph in regard to the signal source 110 in the case of a frequency equality f1=f2 is shown in FIG. 21. RF IN indicates a signal that is input from the signal source 110 to the DUT 200, etc., S11m the S parameter of the DUT 200, etc. obtained from a signal reflected from the DUT 200, etc., and S11a the true S parameter of the DUT 200, etc. without any measurement system errors.
In the case of a frequency equality f1=f2, it is possible to correct errors in such a way as described in, for example, Japanese Patent Laid-Open Publication No. Hei. 11-038054. This kind of correction is called calibration. An outline of calibration will now be given. A correction kit is connected to the signal source 110 to achieve three kinds of conditions of opening, shorting and loading (standard loading Z0). Signal reflected from the correction kit above is acquired by the bridge to obtain three kinds of S parameters (S11m) corresponding to the three kinds of conditions. Then, the three kinds of variables Ed, Er and Es are obtained from the three kinds of S parameters.
However, there can be a case where the frequency f1 is not equal to the frequency f2. For example, the case is where the DUT 200 is a device having frequency conversion functions such as a mixer. In such a case, it is impossible to correct errors in such a calibration as above. A signal flow graph in regard to the signal source 110 in the case where the frequency f1 is not equal to the frequency f2 is shown in FIG. 22. Ed and Es flow in the same manner as the case where the frequency f1 is equal to the frequency f2, while Er is divided into Er1 and Er2. In such a calibration as above are obtained only three kinds of S parameters (S11m), and thus Ed, Es and Er1-Er2 only can be obtained. Therefore, it is impossible to obtain Er1 and Er2.
Further, in the case where the frequency f1 is not equal to the frequency f2, measurement system errors due to the receiving section 120 cannot also be ignored. A signal flow graph in the case of a direct coupling of the signal source 110 and the receiving section 120 is shown in FIG. 23. S2 lm is the S parameter of the DUT 200, etc. obtained from a signal received by the receiving section 120. As shown in FIG. 23, there occur measurement system errors such as Et and EL due to the receiving section 120. These cannot also be obtained in such a calibration as above.
Accordingly, in the case where the frequency f1 is not equal to the frequency f2, measurement system errors cannot be obtained and approximate values including errors are to be measured.
Hence, it is a subject of the present invention to make it possible to correct measurement system errors even in the case where the frequency of an input signal of a device under test is different from that of the output signal thereof.